Sheet processing apparatus and sheet processing method

ABSTRACT

A sheet processing apparatus comprises a first conveyer roller pair and a second conveyer roller pair arranged on a sheet conveyance route along with a punch unit arranged between the first conveyer roller pair and the second conveyer roller pair. When it detects the tail end of a sheet of paper being conveyed in the direction from the first conveyer roller pair toward the second conveyer roller pair passing the first conveyer roller pair, it drives the second conveyer roller pair to rotate reversely and move the sheet of paper backward. Additionally, it causes a stopper to appear at a position defined by a predetermined distance of backward movement of the tail end of the sheet of paper and so as to make the sheet of paper abut the stopper and correct the skew, if any. After correcting the skew, it cuts a hole in the sheet of paper by means of the punch unit.

CROSS REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2005-276595, filed on Sep. 22, 2005, the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a sheet processing apparatus for correcting skews and performing punching processes on the sheets delivered from an image forming apparatus such as an MFP (multi function peripherals), which is a composite digital machine, a copying machine or a printer.

2. Description of the Related Art

Image forming apparatus including MFPs, copying machines and printers are provided at the sheet delivery section of the image forming apparatus main body with a post-processing unit for performing post-processing operations including an operation of sorting sheets where an image is formed, that of stapling sheets and/or that of punching sheets.

In such a post-processing unit, the sheet of paper delivered from the image forming apparatus can move aslant and become skewed relative to the sheet delivering direction. When the skewed sheet of paper is subjected to a punching process (and punched), one or more than one holes can be produced at wrong positions by the punching process to give rise to a problem for a succeeding filing operation. For this reason, the post-processing unit is equipped with a skew correcting means for correcting the skew (if any) of the sheet of paper before the punching process.

Jpn. Pat. Appln. Laid-Open Publication No. 2002-274699 describes a sheet processing apparatus that can correct skews. The apparatus described in the above-cited Patent Document comprises a punch unit for performing a punching process on the sheet of paper being delivered to it and the sheet of paper is made to abut the nipper of the inlet roller arranged upstream of the punch unit in order to correct the skew, if any, of the sheet of paper.

However, with the apparatus described in the above-cited Patent Document, the sheet of paper that has been corrected for the skew, if any, is forced to travel a long distance before it gets to the punch unit. Therefore, the sheet of paper can be skewed again during the travel over that distance.

Jpn. Pat. Appln. Laid-Open Publication No. 2002-193493 describes a punching processing apparatus that can correct the skew, if any, of a sheet of paper. In the punching processing apparatus described in the above cited patent document, a registration member (registration roller) is arranged downstream relative to the punching means of the apparatus and the front end of the sheet of paper being delivered from an image forming apparatus main body is made to abut the registration member by the sheet conveying force of the sheet delivery roller of the image forming apparatus main body in order to correct the skew, if any.

However, since punching processing apparatus described in the above cited patent document utilizes the sheet conveying force of the sheet delivery roller of the image forming apparatus main body, a certain degree of positional accuracy is required for aligning the image forming apparatus main body and the post processing apparatus to by turn poses limitations to the mechanical configuration of the post processing apparatus.

In view of the above identified circumstances, it is therefore the object of the present invention to provide a sheet processing apparatus having a simple configuration that can correct the skew, if any, of a sheet of paper and accurately execute a punching process.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of an image forming apparatus equipped with a sheet processing apparatus according to the present invention, showing the overall configuration thereof;

FIG. 2 is a schematic illustration of an embodiment of sheet processing apparatus according to the present invention, showing the configuration thereof;

FIG. 3 is a schematic block diagram of the control system of a sheet processing apparatus according to the present invention;

FIGS. 4A through 4E are schematic illustration of the operation of a sheet processing apparatus according to the present invention;

FIG. 5 is a schematic illustration of the skew correcting operation of a sheet processing apparatus according to the present invention;

FIG. 6 is a schematic illustration of another embodiment of sheet processing apparatus according to the present invention, showing the configuration thereof; and

FIG. 7 is a schematic illustration of another image forming apparatus equipped with a sheet processing apparatus according to the present invention, showing the overall configuration thereof.

DETAILED DESCRIPTION OF THE INVENTION

Throughout this description, the embodiments and examples shown should be considered as exemplars, rather than limitations on the apparatus of the present invention.

Now, the present invention will be described by referring to the accompanying drawings that illustrate preferred embodiments of the invention. Throughout the drawings, the same or similar components are denoted respectively by the same reference symbols.

FIG. 1 is a schematic illustration of an image forming apparatus equipped with a sheet processing apparatus according to the present invention, showing the overall configuration thereof. FIG. 2 is a schematic illustration of a principal part of an embodiment of sheet processing apparatus according to the present invention, showing the configuration thereof.

FIG. 1 shows an MFP (multi function peripheral) that operates as an image forming apparatus. The image forming apparatus 10 comprises a scanner section 11, an automatic document feeder (ADF) 12 and an operation section 13 in an upper part thereof. A printer section 14 and a sheet feeding section 15 are arranged respectively in a central part and in a lower part of the image forming apparatus 10. A finisher 20 is arranged contiguously relative to the image forming apparatus 10 for punching operations and stapling operations.

The printer section 14 typically has a tandem type color laser printer. The printer section 14 includes a laser 141 and a photosensitive drum 142. A charging device 143, a developing device 144, a transferring device 145 and the like are arranged around the photosensitive drum 142. Toner is supplied to the developing device 144 from a toner cartridge.

Now, the operation of the printer section 141 will be described below. The surface of the photosensitive drum 142 is scanned for exposure by the laser beam from the laser 14 to form an electrostatic latent image on the photosensitive drum 142. The electrostatic latent image is developed by the developing device 144 to form a toner image on the photosensitive drum 142. The toner image is then transferred onto a sheet of paper P by the transferring device 145. For a color printing process, toner cartridges of four colors including cyan, magenta, yellow and black are arranged in a row and photosensitive drums and optical units of the respective colors are provided.

The sheet feeding section 15 has a plurality of sheet feeding cassettes for containing sheets of paper of various different sizes. A sheet of paper P is sent from one of the sheet feeding cassettes to conveyor belt 16 and an image is formed on the sheet of paper P. The sheet of paper P on which an image is formed is then delivered to the outside by way of sheet delivery section 17 and conveyed to the finisher 20.

The finisher 20 performs post-processing operations such as stapling and punching on the sheet of paper P and delivers it to either sheet delivery tray 21 or sheet delivery tray 22. The sheet delivery trays 21, 22 are driven to move up and down by a sheet delivery tray drive section so that the sheet of paper P is delivered to either of the trays.

The finisher 20 is provided with a sheet processing apparatus 30 according to the present invention. The sheet processing apparatus 30 comprises a punch unit 40 for punching the sheet of paper P conveyed from the image forming apparatus 10 and a skew correcting section 50 for correcting the skew of the sheet of paper P before punching the sheet.

FIG. 2 schematically illustrates the specific configuration of the sheet processing apparatus 30. Referring to FIG. 2, reference symbol 40 denotes the punch unit. The punch unit 40 includes a hole cutting blade 41, an eccentric cam 42 for driving the hole cutting blade 41 to move up and down, a punch motor 43 for driving the eccentric cam 42 to rotate and a punch die 44. The punch die 44 includes an upper die 441 and a lower die 442 and the sheet of paper P is conveyed between the upper die 441 and the lower die 442. The sheet of paper P is punched as the hole cutting blade 41 is reciprocated in the direction of the punch die 44 (that is orthogonal relative to the direction of conveying the sheet of paper P). While the eccentric cam 42 is described above as means for reciprocating the hole cutting blade 41, it may be replaced by some other means for reciprocating the hole cutting blade 41.

On the other hand, a first conveyer roller pair 51 and a second conveyer roller pair 52 are arranged respectively at the sheet feeding side and at the sheet discharging side of the punch unit 40. The conveyer roller pairs 51, 52 are driven to rotate respectively by conveyor motors 53, 54. The first conveyer roller pair 51 includes upper and lower conveyer rollers 511, 512, which are driven to rotate by the conveyer motor 53 to convey a sheet of paper P, pinching it between them.

The second conveyer roller pair 52 includes upper and lower conveyer rollers 521, 522, which are driven to rotate by the conveyer motor 54 to convey a sheet of paper P, pinching it between them. The second conveyer roller pair 52 can be driven to rotate reversely by the conveyer motor 54 so that it may convey a sheet of paper P to the delivery tray 21 or 22 or to the other side depending on the sense of rotation thereof.

The second conveyer roller pair 52 can change the condition of contact of the rollers 521, 522 by means of a load controller 55. More specifically, the second conveyer roller pair 52 has a functional feature of registration roller where the rollers 521, 522 are brought into tight contact with each other and a functional feature of slipping roller where the rollers 521, 522 are brought into loose contact with each other.

Additionally, a sensor 56 is arranged between the sheet feeding side of the punch unit 40 and the first conveyer roller pair 51 to detect the condition of being conveyed of the sheet of paper P. In this embodiment of the invention, the sheet of paper P is turned upside down (with the image carrying surface facing downward) when it is conveyed in the direction of arrow X. Any of a variety of mechanisms for turning the sheet of paper P upside down may be used for the purpose of the present invention.

FIG. 3 is a schematic block diagram of the control system of a sheet processing apparatus 30 according to the present invention and an image forming apparatus 10 to be used with the sheet processing apparatus 30. Referring to FIG. 3, reference symbol 61 denotes a control section, which is a microprocessor that includes a CPU. In FIG. 3, reference symbol 62 denotes a motor drive circuit for driving the punch motor 43 and reference symbol 63 denotes a motor drive circuit for driving the conveyer motor 53, whereas reference symbol 64 denotes a motor drive circuit for driving the conveyer motor 54. The control section 61 controls the load controller 55 and the outcome of the detecting operation of the sensor 56 is input to it.

In FIG. 3, reference symbol 65 denotes a control section for controlling the image forming apparatus 10, which is a microprocessor that includes a CPU. The control section 61 and the control section 65 exchange information with each other to coordinate the image forming operation of the image forming apparatus 10 and the operation of the finisher 20. Various directives are input by the user to the control section 65 by way of the operation section 13 and the control section 65 controls the operation of the printer section 14 and that of the sheet feeding section 15 according to the directives so as to convey the sheet of paper P on which an image is formed to the finisher 20.

Now, the operation of the sheet processing apparatus 30 according to the present invention will be described by referring to FIGS. 4A through 4E. The sheet of paper P on which an image is formed by the printer section 14 of the image forming apparatus 10 is then conveyed to the sheet processing apparatus 30. Since the sheet of paper P flows from the printer section 14 toward the tray 21 or the tray 22, the side of the printer section 14 is referred to as upstream side and the side of the tray 21 or the tray 22 is referred to as downstream side in the following description.

FIG. 4A illustrates a condition where the sheet of paper P on which an image is formed by the printer section 14 is being conveyed toward the punch unit 40. In this condition, the first conveyer roller pair 51 and the second conveyer roller pair 52 are driven respectively by the conveyer motors 53, 54 to rotate in the same sense so that the sheet of paper P is conveyed in the first direction (as indicated by arrow X) from the upstream side toward the downstream side. The rotation of the conveyer motor 53 and that of the conveyer motor 54 in this condition are referred to as forward rotation. The hole cutting blade 41 of the punch unit 40 is separated from and located above the punching die 44 in this condition.

As the sheet of paper P is conveyed further toward the downstream side and the sensor 56 detects that the tail end section of the sheet of paper P passes the first conveyer roller pair 51 as shown in FIG. 4B, the motors 53, 54 are stopped and hence the first and second conveyer roller pairs 51, 52 stop rotating. Then, while the first conveyer roller pair 51 is held in a halted state, the rollers of the second conveyer roller pair 52 is made to lightly touch each other by the load controller 55 and driven to rotate in the opposite sense of rotation by the motor 54.

Then, as a result, the sheet of paper P is conveyed in the direction opposite to the direction indicated by the arrow X (second direction) by the second conveyer roller pair 52 and eventually the tail end of the sheet of paper P abuts the first conveyer roller pair 51 as shown in FIG. 4C. In other words, the first conveyer roller pair 51 operates as a stopper for the sheet of paper P that appears at a position defined by a predetermined distance of backward movement of the tail end of the sheet of paper P.

On the other hand, the second conveyer roller pair 52 operates as a slipping roller so as to be driven to rotate in the opposite sense of rotation, while the rollers of the second roller pair 52 are being made to lightly touch the sheet of paper P. Thus, if the sheet of paper P is skewed, the skew is corrected as the sheet of paper P is forced to abut the first conveyer roller pair 51 while it is slightly bent. The sensor 56 detects that the sheet of paper P is driven to move back.

As the sheet of paper P that is being driven to move back is detected by the sensor 56, the second conveyer roller pair 52 is driven to rotate in the opposite sense of rotation by a predetermined time period and made to stop. Then, as shown in FIG. $D, the eccentric cam 42 of the punch unit 40 is driven to rotate by the punch motor 43 and the hole cutting blade 41 is forced to move downward. As a result, the sheet of paper P is subjected to a punching process. Since the skew, if any, of the sheet of paper P has been corrected at the time of the punching process, the sheet of paper P is punched at a right position.

After the punching process, the second conveyer roller pair 52 is controlled by the load controller 55 so as to operates as registration roller. Then, the conveyer roller pairs 51, 52 are driven to rotate forwardly by the respective motors 53, 54 and the sheet of paper P is conveyed in the direction indicted by the arrow X as shown in FIG. 4E so that the conveyer roller pairs 51, 52 become ready for receiving another sheet of paper P. Thereafter, the steps of operation of FIGS. 4A through 4E are repeated so that sheets of paper P are sequentially subjected to a punching process and then delivered to the delivery tray 21 or 22.

Thus, the sensor 56 detects the sheet of paper P when it is fed it, when it passes the first conveyer roller pair 51, when it is driven to move back and when it is forwardly conveyed once again and passes the sensor 56 and each time notifies the control section 61 of what it detects. Then, the control section 61 controls the motor drive circuits 62, 63, 64 and the load controller 55 according to what is detected by the sensor 56.

FIG. 5 is a schematic illustration of the skew correcting process and the punching process of a sheet processing apparatus according to the present invention. FIG. 5 shows a situation where a sheet of paper P has passed the first conveyer roller pair 51 once and then is being driven to move back by the second conveyer roller pair 52 (not shown in FIG. 5).

If the sheet of paper P is skewed as indicated by solid lines in FIG. 5, the skew is corrected as indicated by dotted chain lines in FIG. 5 as it is driven to move back by the second conveyer roller pair 52 and made to abut the first conveyer roller pair 51. Since the sheet of paper P is subjected to a punching process by the punch unit 40 thereafter, the sheet of paper P is punched to produce a hole H at a right position.

Note that the hole cutting position in the punching process is defined typically by the predetermined distance L1 from the tail end p0 of the sheet of paper P as viewed in the sheet conveying direction (e.g., 12 mm). Thus, it is important to define the gap L2 between the first conveyer roller pair 51 and the hole cutting blade 41 as L2≈L1 to take the distance L1 into consideration.

Thus, with a sheet processing apparatus 30 according to the present invention, a sheet of paper P is conveyed from the upstream side toward the downstream side and the skew, if any, of the sheet of paper P is corrected as it is driven to move back when it passes the first conveyer roller pair 51 and made to abut the first conveyer roller pair 51. In other words, the skew, if any, is corrected at the upstream side in the sense of conveying the sheet of paper so that the sheet of paper is subjected to a punching process after having been corrected for the skew so that it is reliably punched at the right position.

FIG. 6 is a schematic illustration of another embodiment of sheet processing apparatus according to the present invention, showing the configuration thereof. Referring to FIG. 6, a shutter 57 is arranged between the sensor 56 and the first conveyer roller pair 51 so as to be driven to move up and down by a motor 58. If the shutter 57 is at the underside of the route of conveying a sheet of paper P, it can be driven to project upward across the route of conveying the sheet of paper P by the motor 58. In other words, the shutter 57 operates as a stopper for the sheet of paper P that appears at a position defined by a predetermined distance of backward movement of the tail end of the sheet of paper P.

Thus, while the sheet of paper P is conveyed backward so as to abut the first conveyer roller pair 51 in the embodiment of FIG. 2, the shutter 57 is made to project across the route of conveying the sheet of paper P and the sheet of paper P that is being conveyed backward is made to abut the shutter 57 to correct the skew, if any. The timing of projecting the shutter 57 across the route of conveying the sheet of paper P is made to agree with the time when the sensor 56 detects that the tail end p0 of the sheet of paper P passes the position of the sensor 56 (immediately after the situation of FIG. 4B), whereas the timing of retracting the shutter 57 is made to agree with the time when the punching process comes to end (immediately before the situation of FIG. 4E).

For the purpose of controlling the forward and backward movements of the shutter 57 with regard to the route of conveying the sheet of paper P, a motor drive circuit 66 for driving the motor 58 is provided and controlled by the control section 61 and the operation of the shutter 57 is controlled in response to the outcome of detection of the sensor 56 as shown by dotted line in FIG. 3.

Thus, as a result of providing a shutter 57, the sheet of paper P reliably abuts it when it is driven to move backward so that it is possible to highly reliably correct the skew, if any, of the sheet of paper P.

In the instance of FIG. 1, a finisher 20 is arranged contiguously relative to the image forming apparatus 10 and provided with a punch unit 40. However, the punch unit 40 may alternatively be contained in the image forming apparatus 10. More specifically, as shown in FIG. 7, the printer section 14 and the sheet processing apparatus 30 may be arranged in a main body of the image forming apparatus 10 and the sheet of paper P delivered from the printer section 14 may be subjected to a hole cutting process by the punch unit 40 and then to a skew correcting process.

Thus, with a sheet processing apparatus according to the invention as described above, the sheet of paper being conveyed from the upstream side to the downstream side is temporarily driven to move back and made to abut the stopper in order to correct the skew, if any, thereof before it is subjected to a punching process after the skew correction. Therefore, a sheet processing apparatus according to the present invention can always punch a sheet of paper at the right position.

Although exemplary embodiments of the present invention have been shown and described, it will be apparent to those having ordinary skill in the art that a number of changes, modifications, or alternations to the invention as described herein may be made, none of which depart from the spirit of the present invention. All such changes, modifications, and alterations should therefore be seen as within the scope of the present invention. 

1. A sheet processing apparatus for executing a hole cutting process on a sheet of paper carrying an image formed thereon, the apparatus comprising: a sheet conveying section including a first conveyer roller pair and a second conveyer roller pair arranged on a sheet conveyance route and adapted to convey a sheet of paper along the conveyance route by the rotation of the first conveyor roller pair and that of the second conveyor roller pair; a punch unit arranged between the first conveyer roller pair and the second conveyer roller pair and adapted to execute a hole cutting process on the sheet of paper; a drive section that controls the rotary condition of the first conveyer roller pair and that of the second conveyer roller pair so as to convey the sheet of paper in the first direction from the first conveyer roller pair to the second conveyer roller pair or reversely in the second direction from the second conveyer roller pair to the first conveyer roller pair; and a skew correcting section that drives the second conveyer roller pair to rotate reversely on the way of conveying the sheet of paper in the first direction after the time when the tail end of the sheet of paper passes the first conveyer roller pair to move the sheet of paper backward in the second direction, causing a stopper to appear at a position defined by a predetermined distance of backward movement of the sheet of paper so as to make the backwardly moving sheet of paper abut the stopper in order to correct, if any, of the skew of the sheet of paper; the first conveyer roller pair and the second conveyer roller pair being so controlled as to convey the sheet of paper in the first direction after cutting a hole in the sheet of paper by the punch unit after correcting the skew, if any, thereof.
 2. The apparatus according to claim 1, wherein the first conveyer roller pair is made to operate as the stopper and adapted to pinch a sheet of paper between the rollers of the pair and convey it; and the skew correcting section stops the rotary motion of the first roller pair and makes the backwardly moving sheet of paper abut the first conveyer roller pair in order to correct the skew, if any, of the sheet of paper when the sheet of paper is driven to move backward.
 3. The apparatus according to claim 1, wherein the stopper of the skew correcting section is formed by a shutter adapted to project across the route of backward movement of the sheet of paper in the second direction and the shutter is driven to project across the conveyance route so as to make the sheet of paper abut the shutter in order to correct the skew, if any, of the sheet of paper when the sheet of paper is driven to move backward.
 4. The apparatus according to claim 3, wherein the shutter is adapted to be driven by a drive motor so as to retractably project across the route of backward movement of the sheet of paper in the second direction.
 5. The apparatus according to claim 1, further comprising: a sensor that detects the tail end of the sheet of paper being conveyed in the first direction and passing the first conveyer roller pair and a backward movement of the sheet of paper in the second direction, wherein the skew correcting section controls the drive section by using the outcome of the detecting operation of the sensor and controls the rotary condition of the first conveyer roller pair and that of the second roller pair.
 6. The apparatus according to claim 1, wherein the second conveyer roller pair is adapted to pinch a sheet of paper between the rollers of the pair and convey it; and the skew correcting section causes the rollers of the second conveyer roller pair to lightly contact with each other and operate as slipping rollers when conveying the sheet of paper in the second direction.
 7. The apparatus according to claim 1, wherein the hole cutting position of the sheet of paper is determined by the gap between a hole cutting blade of the punch unit and the stopper; and the punch unit is adapted to execute a hole cutting process near the tail end of the sheet of paper corrected for the skew, if any.
 8. The apparatus according to claim 7, wherein the gap L2 between the hole cutting position by the punch unit and the stopper is defined to be L2≈L1, where L1 is the gap between the tail end of the sheet of paper and the hole cutting position.
 9. A sheet processing apparatus for executing a hole cutting process on a sheet of paper carrying an image formed thereon, the apparatus comprising: a sheet conveying section including a first conveyer roller pair and a second conveyer roller pair arranged on a sheet conveyance route and adapted to convey a sheet of paper along the conveyance route by the rotation of the first conveyor roller pair and that of the second conveyor roller pair; a punch unit arranged between the first conveyer roller pair and the second conveyer roller pair and adapted to execute a hole cutting process on the sheet of paper; a drive section that controls the rotary condition of the first conveyer roller pair and that of the second conveyer roller pair so as to convey the sheet of paper in the first direction from the first conveyer roller pair to the second conveyer roller pair or reversely in the second direction from the second conveyer roller pair to the first conveyer roller pair; a skew correcting section that drives the second conveyer roller pair to rotate reversely on the way of conveying the sheet of paper in the first direction after the time when the tail end of the sheet of paper passes the first conveyer roller pair to move the sheet of paper backward in the second direction, causing a stopper to appear at a position defined by a predetermined distance of backward movement of the sheet of paper so as to make the backwardly moving sheet of paper abut the stopper in order to correct, if any, of the skew of the sheet of paper; and a control section including detection means for detecting the position of the sheet of paper on the conveyance route and adapted to control the rotary condition of the first conveyer roller pair and that of the second conveyer roller pair, the appearance of the stopper and the punching process of the punch unit in response to the outcome of the detection means; the sheet of paper corrected for the skew, if any, being subjected to a hole cutting process and delivered.
 10. The apparatus according to claim 9, further comprising: a sensor arranged in close proximity of the first conveyer roller pair to detect the position of the sheet of paper on the conveyance route.
 11. A sheet processing method comprising: arranging a first conveyer roller pair and a second conveyer roller pair on a sheet conveyance route and a punch unit between the first conveyer roller pair and the second conveyer roller pair for executing a hole cutting process on the sheet of paper; controlling the rotary condition of the first conveyer roller pair and that of the second conveyer roller pair so as to convey the sheet of paper in the first direction from the first conveyer roller pair to the second conveyer roller pair; reversing the sense of rotation of the second conveyer roller pair to drive the sheet of paper to move backward reversely in the second direction from the second conveyer roller pair to the first conveyer roller pair on the way of conveying the sheet of paper in the first direction at the time when the tail end of the sheet of paper passes the first conveyer roller pair; causing a stopper to appear at a position defined by a predetermined distance of backward movement of the sheet of paper so as to make the backwardly moving sheet of paper abut the stopper in order to correct, if any, of the skew of the sheet of paper; cutting a hole in the sheet of paper by means of the punch unit after correcting the skew, if any, thereof; and controlling the sense of rotation of the first conveyer roller pair and that of the second conveyer roller pair so as to convey the sheet of paper in the first direction after cutting a hole in the sheet of paper.
 12. The method according to claim 11, wherein the first conveyer roller pair is made to operate as the stopper and adapted to pinch a sheet of paper between the rollers of the pair and convey it; and the rotary motion of the roller pair is stopped and the backwardly moving sheet of paper is made abut the first conveyer roller pair in order to correct the skew, if any, of the sheet of paper when the sheet of paper is driven to move backward in the second direction.
 13. The method according to claim 11, wherein the stopper is formed by a shutter adapted to project across the route of backward movement of the sheet of paper in the second direction and the shutter is driven to project across the conveyance route so as to make the sheet of paper abut the shutter in order to correct the skew, if any, of the sheet of paper when the sheet of paper is driven to move backward.
 14. The method according to claim 13, wherein the shutter is adapted to be driven by a drive motor and made to project across the route of backward movement of the sheet of paper in the second direction when correcting the skew, if any.
 15. The method according to claim 11, wherein the second conveyer roller pair is adapted to pinch the sheet of paper between the rollers of the pair and convey it; and the rollers of the second conveyer roller pair are caused to lightly contact with each other and operate as slipping rollers when conveying the sheet of paper in the second direction at the time of correcting the skew, if any. 